In the past decade considerable effort has gone into the development and characterization of perfluorosulfonic acid polymer electrolytes such as Nafion. These efforts have shown that polymer electrolyte membranes (PEM) offer a number of advantages over conventional electrolytes when used in electrochemical devices such as fuel cells and water electrolyzers. Unfortunately, these electrolytes must remain hydrated to retain ionic conductivity, which limits their maximum operating temperature to 100.degree. C. at atmospheric pressure.
This disadvantage of known PEM materials, therefore, is highlighted in those systems in which a polymer electrolyte with high conductivity at temperatures in excess of 100.degree. C. would be useful. One such application is the H.sub.2 /O.sub.2 fuel cell that utilizes reformed hydrogen from organic fuels (methane, methanol, etc.) which will have a certain amount of CO that poisons the electrode catalysts. Another such application is the direct methanol fuel cell. Present direct methanol-air fuel cell configurations are severely limited by the lack of sufficiently active catalysts for the methanol anode, and to a lesser extent, the oxygen cathode. This is a direct result of catalyst poisoning caused by carbon monoxide produced by the fuel at operating temperatures of about 100.degree. C. or lower.
Another disadvantage of known PEM methanol-air fuel cells is seen in poor performance of the fuel cells due to the high rate of methanol cross-over from the anode to the cathode through the membrane, which results in a loss of efficiency via chemical reaction of the fuel with oxygen and consequent depolarization of the cathode.
The use of solid polymer electrolytes offers new opportunities to overcome these catalyst stability and activity problems, provided the polymers selected are stable and retain reasonable ionic conductivity at temperatures approaching 200.degree. C., avoiding anode/cathode poisoning effects. Further, such polymers should have other desirable properties, such as low methanol permeability to reduce the efficiency losses resulting from crossover.
It has now been discovered that films comprising polymers containing basic groups that can form complexes with stable acids or polymers containing acidic groups provide a viable alternative to known PEM's and other conventional electrolytes. Polybenzimidazole (PBI) which has been doped with a strong acid, such as phosphoric acid or sulfuric acid, is an example of a suitable polymer. Polybenzimidazoles, along with other suitable aromatic polymers, basic enough to complex with acids, exhibit excellent oxidative and thermal stability characteristics, these properties being further enhanced by doping at a level of at least 200 mol %. They require low water activity, thus avoiding operating temperature limits due to the boiling point of water. Capability to operate at elevated temperatures, i.e. up to at least 200.degree. C., also reduces the potential for anode/cathode poisoning. Further, they do not suffer significantly from methanol cross-over because of low methanol swelling with methanol vapor and high glass transition temperatures.
It is, therefore, an object of the subject invention to provide a solid polymer electrolyte which does not suffer from known problems associated with catalyst stability and activity.
It is another object of the invention to provide a solid polymer electrolyte which is stable and retains reasonable ionic conductivity at up to at least 200.degree. C.
It is still another object of the invention to provide a solid polymer electrolyte which is suitable for use in direct methanol fuel cells without exhibiting high methanol permeability resulting in loss in efficiency due to methanol crossover.